1. Field of the Invention
The invention relates to a hydraulic power steering system.
2. Discussion of Background
There is a conventional hydraulic power steering system that generates steering assist force by supplying hydraulic fluid from a hydraulic pump to a power cylinder, which is coupled to a steering mechanism of a vehicle, via a hydraulic control valve. In a commonly-used hydraulic power steering system, a hydraulic control valve is mechanically coupled to a steering member, such as a steering wheel, via a steering shaft, and the opening degree of the hydraulic control valve is controlled in response to an operation of the steering member.
Japanese Patent Application Publication No. 2006-306239 (JP 2006-306239 A) describes a hydraulic power steering system in which the opening degree of a hydraulic control valve is controlled with the use of an electric motor (valve driving motor) without mechanical coupling between the hydraulic control valve and a steering member.
In the hydraulic power steering system in which the opening degree of the hydraulic control valve is controlled by the valve driving motor, when the ambient temperature of the hydraulic control valve is low, the viscosity of hydraulic fluid inside the hydraulic control valve becomes high or the friction of an oil seal inside the hydraulic control valve becomes large. As a result, motor torque of the valve driving motor becomes insufficient, leading to reduction in response of the hydraulic control valve to the opening degree control. This causes deterioration of the steering feel.